Hydrostatic bending and die forming

ABSTRACT

A process and apparatus for bending and forming an elongated workpiece into an article of defined cross-sectional shape which consists of pulling the workpiece continuously through a sequence of dies disposed to bend and form the workpiece to the final desired shape while subjecting the surface of the workpiece to fluid hydrostatic pressure.

Unt Stats Inventors Air/in 1M1. Salbmii Columbus; Robert .11. lFiorentino, Worthington, both of,

. Ohio Filed May 8, 1970 Patented Aug. 31, 11971 Assignee The Bottelie Development Corporation Collnmlous, tOhio Continuation 01 application Ser. No. 153?;5332NQY: 71.126?! nowiariqm HYDROSTATMC BENDING AND lDtlIlE FORMING 6 Cllairns, 4 Drawing Figs.

US. Cl 72/60, 72/176, 72/282 llnt. C1 1321c 3/10 [50] Field oi Search 72/60, 253, 710, 282, 176

[56] References Cited UNITED STATES PATENTS 2,977,630 4/1961 Bazler 18/4 3,328,998 7/1967 Sabroff et al. 72/60 3,344,636 10/1967 Pugh 72/60 3,379,043 4/1968 Fuchs, Jr. 72/60 3,415,089 12/1968 Ferchland 72/60 Primary Examiner-Richard J. Herbst AttorneyGray, Mase and Dunson ABSTRACT: A process and apparatus for bending and forming an elongated workpiece into an article of defined crosssectional shape which consists of pulling the workpiece continuously through a sequence of dies disposed to bend and form the workpiece to the final desired shape while subjecting the surface of the workpiece to fluid hydrostatic pressure.

HYDROSTATIC BENlDllNG AND DE FQRMIING This is a continuation of Ser. No. 683,522, filed Nov. 16, 1967, now abandoned.

BACKGROUND This invention relates to a new and improved means for bending and forming strip and relates in particular to a new and improved method and apparatus for continuous bending and forming structural members from metal strip.

In the airframe and space vehicle industry, there is a great demand for structural members fabricated of lightweight metals such as titanium and aluminum. Among these members are those of cross sectioned Z, L, U, and hat-shaped configurations. Commonly employed methods for obtaining such members include extruding, roll forming and brake forming.

Extruding is an expensive means for obtaining such members particularly where the members are to be fabricated from high-strength materials such as titanium alloys in that not only is it necessary to employ elaborate and expensive extrusion equipment and dies but it also may be necessary to size and finish the extruded members by cold or warm drawing. Extruded structural members frequently do not meet the necessary surface finish and size tolerances due to the nature of the hot-extrusion process. Consequently, it is common practice to draw an extruded structural member through a sizing and surfacing die to obtain a usable product. However, even this additional elaborate step often fails to produce a satisfactory member in that the stress effects of such a draw may effect a warp or bend in the final product. Further, the cold-working and annealing procedures associated with such a process can adversely affect the strength properties of the final product.

Simple shapes such as described above may be formed by bending metal strip into the desired shape. This may be accomplished by passing a metal strip between a series of appropriately positioned shaped rolls to bend the strip into the desired shape in a continuous fashion or the strip may be bent into the desired shape by means of a press brake. Although such means are effective to economically produce such shapes, all metal strip is severely limited in the complexity of configurations and the sharpness of bend obtainable by such procedures. For example, with titanium alloys, experience to date indicates that limitations in both equipment and formability require that shapes roll-formed at ambient or low temperatures must have very generous corner radii, at least 3T to 6T (where T is the sheet thickness). The recommended bend radius for most high-strength aluminum alloy strip of from 0.012 to 0.073 inch gage is from A; to 7/16 inch (see ASM Hand-book, 8th Edition, Vol. 1, pg. 982). If such bend radii are exceeded, and often even when not exceeded, checks and cracks occur along the outside surface of the bend.

Such checks and cracks are, of course, unacceptable, since any crack on the surface of a structural member subjected to stress and vibration, typical of aircraft and space vehicle application, are susceptible to crack propagation and failure.

On the other hand the round bends or the large radii required in forming these members by the known bending techniques materially reduce the efficiency of such structural members.

We have now found that through the utilization of a hydrostatic pressure technique, it is possible to effect metal strip forming into sharp cornered, mechanically sound and surface defect-free structural shapes of such cross-sectional configurations as Z, L, U, and hat shapes through a continuous bending and forming procedure.

INVENTION In general, the present invention consists of a method for forming strip which consists of pulling the strip through at least one die opening while subjecting the surface of the metal to hydrostatic pressure of a magnitude to prevent any surface rupture. The present invention also consists of a preferred apparatus consisting of a pressure chamber provided with a series of dies with die openings disposed to bend and form strip pulled through the chamber.

FIG. 1 of the drawings is a perspective view of a pressurechamber multiple-form die apparatus shown partially in cross section that is in conformity with the preferred apparatus of the present invention. The apparatus is shown as utilized to form metal strip into a structural conformity with the process of the present invention.

FIG. 2 is a cross-sectional view of a broken away portion of an entry orifice which may be used in conjunction with end plates 34 and 36 of the device of FIG. 1.

FIG. 3 is an enlarged broken away perspective view of a positioning sleeve for positioning the dies of the apparatus of FIG. 1.

FIG. 43 is a perspective view of a split die such as may be utilized in conjunction with the process of the present invention.

Metals ordinarily employed in the fabrication of structural members (steel, magnesium, zirconium, beryllium, etc., as well as titanium and aluminum) possess defined mechanical properties in terms of their ability to flow plastically without rupture. The extent to which deformation can be accomplished in a single ambient or low temperature forming step is limited by these mechanical properties. A metal-forming step involving bending at ambient or low temperatures is particularly severe in terms of localized plastic flow along the outside surface of the bend. The reason for this is that the metal at inside surface is subjected to large compressive stresses while the metal near the outside surface of the bend is subjected to large tensile stresses. Accordingly, if the bend is too sharp, checks or cracks occur along the outside surface of the bend because the tensile stresses in these areas exceed the tensile strength of the metal. I

The plastic formability of such metals as determined by their plastic flow or ductile characteristics is further complicated by the fact that if deformation is accomplished at ambient or relatively low temperatures the metal becomes harder or work hardens." Work-hardened metals are particularly resistant to plastic flow. Thus, it is not possible under ordinary circumstances to form or bend a metal beyond its normal plastic flow limitations by sequential steps of forming or bending without stress relieving or softening intermediary heat treatments. Such heat treatments are time consuming, expensive and frequently detrimental to the ultimate surface condition of the formed part.

The application liquid hydrostatic pressure to the surface of solid metal bodies being drawn through a die increases the capacity of the metal for deformation. This is the basic discovery of P. W. Bridgman and is set forth in his publication, Studies in Large Plastic Flow and Fracture," McGraw-I-Iill 1952.

The effect of liquid hydrostatic pressures on a solid metal body simultaneously hydrostatically expressed and drawn through a die within a closed container is disclosed by U.S. Pat No. 2,558,035 to P. W. Bridgman. This method and apparatus differs from the present method and apparatus in that in the present method and apparatus metal strip or sheet passes through the container continuously to be progressively formed or bent by one or more dies while in the method and apparatus of the patent a billet is reduced in cross section entirely within the container. Additionally the method of Bridgman relates essentially to the deforming of materials by extrusion wherein the present process consists essentially of bend forming.

Another patent of interest in hydrostatic metal working is U.S. Pat. No. 524,509 to J. Robertson. By the method and zip paratus of this patent, wire is drawn through a chamber enclosed at either end by a die and is reduced in gage by liquid hydrostatic pressure within the chamber. However, as in the case of the Bridgman disclosures, there is no suggestion of bending and forming strip by the use of one or more dies within a pressure chamber as in the present method and apparatus.

DESCRIPTION The present invention is best illustrated by reference to the drawing wherein a cylindrically shaped pressure chamber 10 is provided with dies 12, l4, l6, l8, and 20 positioned therein. The die orifices 22, 24, 26, 28, and 30 of 12-20 are positioned axially in line with one another and are shaped to progressively bend and form strip 32 as it is pulled through the chamber I in the direction of the arrow from a relatively flat cross section (orifice 22) to a slightly bent or curved cross section (orifice 22) to a greater bent or curved cross section (orifice 26) and to a still greater bent or curved cross section (orifice 28) to the final die orifice (orifice 30) through which strip 32 emerges from the chamber 10. The final orifice 30 forms the strip 32 into a sharp cornered V-shaped structural member.

Chamber is provided with end plates 34 and 36 which are fixed to the open ends of chamber 10 by means of bolts or clamps. O-ring gaskets 42 and 43 positioned in circumferential grooves in dies 12 and 20 completes the hydrostatically sealed integrity of chamber 10.

Dies 12-20 are held together and spaced so that their orifice axes coincide by means of connecting rods 45 and retaining sleeves 44. Each rod 45 is extended through holes in each die, the holes being positioned so that the die orifices may be axially aligned for sequential bending or forming. Sleeves 44 serve to space and align the dies as a unit that generally will be assembled outside of the chamber 10. Thus, die assemblies having more or fewer dies than that shown or having die orifices designed for the sequential forming of members other than V-shapes may be assembled and inserted into the chamber 10.

Positioning sleeves 44 (see FIG. 3) slide over connecting rods 45. One sleeve 44 is positioned on either side of each positioned die and setscrew 47 is drawn up tightly to hold the die in position.

End plates 34 and 36 are bolted to end flanges 11 formed in the ends of cylindrical chamber 56 (see FIG. 1). Each plate is provided with a central opening 48 which expose orifices 30 and 22 of dies 20 and-l2, respectively, for pulling a workpiece such as strip 32 through chamber 10 (and dies 12-20). If strip 32 and orifice 22 have close tolerances (fit tightly) a pressurized liquid within chamber 10 will not leak from orifice 22 unless the pressure is high or the viscosity of the liquid is very low. This is due to the fact that the strip entering chamber 10 7 through orifice 22 tends to oppose liquid flow in a reverse direction. In many instances it will be desirable to take a light draw particularly (reduction in gage) with orifice 30 of die 20 to prevent excessive fluid leakage. Such a draw may be less than 1 percent.

In instances where it is not desirable to seal the chamber 10 by means of tight fitting orifices 22 and 30, or it is desired to avoid taking even a light draw, the openings 48 of end plates 34 and 36 may be made to conform to the cross-sectional configuration of the entering and exiting workpiece and sealing may be effected by means of plastic sealing glands such as are depicted by FIG. 2. FIG. 2 is a cross-sectional view of end plate 36 shown as provided with an orifice 60 which is in substantial conformity with the cross-sectional dimensions of strip 32. One or more rubber or elastomeric plastic strips 62 and 64 are positioned within groove 66 formed around the inner circumference of orifice 60. Passageways 70 and 72 lead from groove 66 to the surface of plate 36 facing the inside of chamber 10. Thus when strip 32 is drawn through orifice 60 in the direction of the arrow (FIG. 2) pressure within chamber 10 will flow through passageways 70 and 72 to groove 66 to cause resilient members 62 and 64 to bear against strip 32 to prevent excessive leakage. It will be appreciated that the higher the pressure that is utilized within chamber 10 the more positive will be the sealing effected by resilient members 62 and 64.

Stem 50 of chamber 10 leads to a liquid pressure source. Such source will generally consist of a hydraulic ram or high pressure pump. The means for connecting stem 50 to such a pressure source not being part of the present invention but falling well within the skill of the art is not shown. However, the lower portion of the piston of a hydraulic ram 52 is shown as extending into the stem as an illustrative means for providing hydrostatic pressure to the interior of chamber 10.

The embodiment thus described is a completely operative embodiment of the present invention. The remaining structure shown by the drawing and not yet described constitutes an improved modification and may be ignored in the basic description.

In the preferred method of starting up and utilizing the 6Al-ayV of the drawings, the die unit comprising dies 12-20 are assembled on positioning rods and are positioned for sequential die forming through their orifices by positioning sleeves 44 outside of die chamber 10. Preferably the dies are threaded prior to inserting them into the pressure chamber and in instances where the strip to be formed is relatively hard or brittle it is most convenient to utilize a more ductile metal lead. For example, where the strip is of Ti-6Al-4V (6 percent aluminum 4 percent Vanadium, balance titanium) it is advisable to weld a more ductile titanium strip as a lead to the Ti-6 Al-4V strip. It may be necessary to chem mill (reduce the gage of the lead by chemical milling) the lead in order to enable one to thread the dies with hand tools. Once the dies are threaded, however, the last portion of the strip preferably will not be drawn completely through an assembled set of dies but should be cut off and left in the dies as a lead for subsequent use.

A particularly useful embodiment to minimize the problem of threading the dies is shown by FIG. 4. The die of FIG. 4 is shown to be segmented but bolted together by bolts 74. If all of the dies are so segmented the die assembly can be split into two parts making it easier to initially thread the die assembly.

End plate 36 is removed and the threaded assembly is slid into chamber 10 (In the manner shown by FIG. 1). End plate 36 will then be replaced (not absolutely necessary in all instances since O-ring 43 seals chamber 10), and the lead portion of strip 32 extending from orifice 30 is attached to pulling means (i.e., conventional grippers).

The cavity of chamber 10 is now filled with a suitable fluid. Generally this will be an oil such as castor oil or petroleum oil; however, the character of such a fluid may vary widely and the optimum fluid as measured in terms of viscosity, etc., will vary with the parameters such as the mechanical properties of the metal strip, the configuration of structural member being formed, etc.

We have found suitable liquids to be those described in our U.S. Pat. No. 3,328,998 that are used in conjunction with hydrostatic extrusion-drawing. We do not exclude the use of any fluid for this application, however, and include gases since one primary use of these materials is to provide pressure on the surface of the strip being formed.

Assuming the chamber 10 to be full of oil, including stem 50, pressure is now brought to bear by means of pressure source. In the apparatus of the drawing, it may be presumed that the ram head 52 is caused to be driven downwardly so as to effect liquid pressure within chamber 10 and elfect hydrostatic pressure on the surface of strip 32. Pulling is now commenced (as with a draw bench) so that the strip 32 is progressively sequentially bent and formed into a sharp-cornered V- shaped structural member as it emerges from orifice 30 of die 20 and while being subjected to high hydrostatic pressures.

Any positive pressure above atmospheric brought to bear on the strip 32 during the progressive bending and forming operation is helpful in reducing cracking or checking along the bend of the formed member, the exact amount of pressure needed to eliminate all cracking and checking will vary. Pressures of at least 5,000 p.s.i. 66 are preferred; however, pressures as high as 400,000 p.s.i. may be necessary.

The pressures within chamber 10 between dies 12-20 are uniform because the dies 14-18 are provided with openings 54 the dies.

Hydrostatic pressure may be supplied to chamber by conduits other than stem 50. In fact it may be desirable to minimize the cross-bore effect caused by stem 50 by providing a conduit through end plate 36.

Although in accordance with the apparatus and method of the present invention the die orifices for sequential forming are axially aligned such axial alignment will not be interpreted as being precise. Such alignment may deviate to such an extent it does not impose undesirable stress properties into the strip.

The method and apparatus of the present invention is described in terms of multiple-forming dies (such as dies 12-20 of FIG. 11) positioned within a pressure chamber (chamber 10 of FIG. 1). Although a plurality of dies is preferred, the number of dies required to effect a given forming operation is dependent upon the degree and completivity of bend or bends to be effected, the mechanical properties of the material to be bent, the fluid pressures employed, etc. Consequently, it may be desirable to employ a multiplicity of forming dies or a single die may be effective. For example, a single die within chamber 10 wherein plates 34 and 36 are provided with sealing entrance and exit orifices such as shown by F IG. 2 may be used or such entrance and exit orifices may simply be "tight fitting" passageways that deter or prevent the leakage of fluid from chamber 10.

We have found that chambers such as chamber 110 are severely limited in the amount of hydrostatic pressure they will withstand without failure because of the stress concentration effects introduced by cross bore or intersecting fluid passages. To provide for higher pressures we enclose chamber 10 within a second similarly shaped chamber 56. End plates 34 and 36 overlap chamber 10 and are bolted to chamber 50 so as to seal this chamber. I

Chamber 56 is provided with a fluid pressure source similar to chamber 110 through a stem 66 which in the embodiment shown surrounds stem 50 of chamber 10. Fluid pressure is represented as being provided by a sleeveshaped plunger 60 forced downwardly by a hydraulic press (not shown).

The value of chamber 56 is in raising or substantially doubling the pressure capabilities of chamber 110. For example, if the pressure integrity of chamber 10 is such that it will safely withstand a total pressure of 200,000 p.s.i. but the pressure requirements for providing a substantially crack-free bend requires 300,000 p.s.i. a pressure of 300,000 p.s.i. may be utilized by providing a fluid pressure within chamber 56 between the walls of chambers 56 and chamber 110) of at least 100,000 p.s.i. The pressure around chamber 10 enables one to raise the pressure within chamber 10 to above the maximum pressure it will tolerate without actually exceeding that maximum pressure.

It will be understood that the fluid used within chamber 56 may be the same as that used within chamber 10 or may differ materially therefrom.

In the forming and bending of metal strip a factor to contend with is springback. Where the last die or where the strip 32 is pulled through a die orifice such as orifice 30 that does not effect any reduction in gage in many instances (as where metal 32 is a relatively hard or resilient metal such as full hard AISI Type 30] stainless steel) the metal strip does not retain a sharp angle but springs back to some extent to a rounded angle or corner. However, if a slight draw is affected by the last die (i.e., 2 percent or less) the bend is sharp and permanent. Consequently, in many instances it may be preferred to effect at least a slight draw of 2 percent or less with the last die.

It will be appreciated that the method of the present invention is primarily concerned with the forming of strip by bending rather than by drawing. For this reason we refer to the means of forcing the strip through the dies as pulling. However, as has been set forth in detail above, in many instances a draw of up to 2 percent is preferred although such a draw is incidental to the primary purpose of effecting a sharp bend.

Although the description of the present invention emphasizes the forming and bending of metal strip, the

method of the present invention is applicable to materials other than metals. It is well known that the problems of cracking when bending any strip material is equivalent to the problems encountered when bending metals. The application of fluid pressure to the surface of resin or plastic strip (such as polyethylene strip) during bending; or die forming reduces bend checking and cracking.

The method and apparatus of the present invention is applicable for forming any point generatrix shape. Thus, for the purpose of the present description and claims the term strip" includes any flat elongated product which can be formed by bending. The term strip" shall also include tubing which can be pulled through the dies of the apparatus of FIG. l and formed into various configurations including cross-sectional complex configurations wherein grooves are formed into the tube surface to effect starlike cross-sectional configurations. In utilizing the apparatus to form tubing it may be desirable to utilize a mandrel such as a floating mandrel or to fill the tubing with a filler material such as a resin or plastic to prevent undue collapse of the tubing wall due to the effects of high hydrostatic fluid pressures. It also may be desirable to fill tubing with a removable metal core. For example, when pull forming and bending titanium tubing the tubing may be filled with copper which may be selectively removed by preferential acid dissolution after forming.

The data of Table 1 (below) relates to bending trials conducted on 0.041 inch gage Ti-6AI-4V (annealed, RC=34) under various hydrostatic pressure environments. Particular attention should be paid to specimens 5 and 6 wherein the minimum bend radius obtainable without failure is shown to be between 3.5 and 3.8T (fora 75 bend die) at atmospheric pressure. However, where a hydrostatic pressure of 100,000 p.s.i. is superimposed (Specimens 3, 7, 8 and 9), it was possible to obtain satisfactory bends with a 1.5l 'll die radius (where Tis the sheet thickness).

TABLE 1 Hydrostatic bend trial results material flAl-4V-Ti in annealed condition RC=34 material thickness 0.041 inch cold. rolled annealed and pickled surface Die Hydrostatic band Die Part Spring Part Specimen pressure, angle, bond bend back, condi- Number p.s.i. 1,000 degrees radius radius degrees tion I F=Iailure (cracked); G=good (no discernible checksot' cracks) M It will be understood that the specific embodiments set forth panying claims are not limited thereby.

1. A process for forming and bending strip material into a shape having a point generatn'x cross section by drawing said strip material through a series of spaced die orifices comprismg:

a. axially aligning the orifices of a plurality of spaced dies of varying orifice shape ranging between the initial crosssectional shape of said strip material to and including the final shape with said point generatrix cross section, said alignment being sequential from the die having an orifice most closely approximating the initial cross-sectional shape of said strip material to the final shape with said point generatrix cross section; and

b. providing a pressurized liquid surrounding the entire sequence of dies providing one equilibrium hydrostatic pressure in the range of 5,000 p.s.i. to 400,000 p.s.i. over the entire length of said strip material in addition to the forming and bending pressures mechanically induced by the working faces of said dies; and c. pulling said strip material continuously through the orifices of said dies to form the strip into the point generatrix cross-sectional shape, said strip passing sequentially from said die orifices most closely approximating the initial shape of said strip to said die substantially of said point generatrix shaped cross section while liquid hydrostatic pressure is applied to the entire length of said strip while being continuously formed into said shape of point generatrix cross section 2. The method of claim 1 wherein at least one die of substantially said point generatrix shape is disposed to effect a draw reduction not more than 2 percent.

3. The method of claim 1 wherein said workpiece consists of a strip of metal and said dies are disposed to sequentially form said strip into a sharp angled structural member of L, V, U, Z,

or hat-shaped configuration.

4. An apparatus for forming and bending strip material into an article with a point generatrix cross section comprising:

a. a chamber;

b. a plurality of dies of varying orifice shape ranging between the initial cross-sectional shape of said strip material to and including said point generatrix cross section aligned within said chamber, said alignment being sequential from the die having an orifice most closely approximating the initial cross-sectional shape of said strip material to a die substantially of said point generatrix cross-sectional shape;

c. meansfor the orifices of the end dies of said alignment to communicate through the walls of said chamber so that said strip material may be pulled through said chamber and said die orifices;

d. means for providing hydrostatic pressure within said chamber to subject said strip material to said hydrostatic pressure along its entire length in addition to the mechanically induced'forming and bending pressures exerted by the working faces of said die orifices while being pulled through said chamber and orifices and sequentially formed into said point generatrix cross-sectional shape; and

e. means for pulling said strip material through said chamber and said plurality of dies to form said strip into the point generatrix cross-sectional shape while liquid hydrostatic pressure is applied to the entire surface of said strip material in addition to the mechanical forming and bending forces exerted by said dies.

5. The apparatus of claim 4 wherein said means for providing liquid hydrostatic pressure within said chamber is capable of providing said pressure within the range of from about 5,000 p.s.i. to 400,000 psi.

6. An apparatus as set forth in claim 4 wherein at least one of said dies is disposed to effect a draw reduction on said point generatrix shape of not more than 2 percent.

W E l Auqust 31,.

Dated Patent No.

Alvin M, Sabrnff Invenwris) and 

1. A process for forming and bending strip material into a shape having a point generatrix cross section by drawing said strip material through a series of spaced die orifices comprising: a. axially aligning the orifices of a plurality of spaced dies of varying orifice shape ranging between the initial crosssectional shape of said strip material to and including the final shape with said point generatrix cross section, said alignment being sequential from the die having an orifice most closely approximating the initial cross-sectional shape of said strip material to the final shape with said point generatrix cross section; and b. providing a pressurized liquid surrounding the entire sequence of dies providing one equilibrium hydrostatic pressure in the range of 5,000 p.s.i. to 400,000 p.s.i. over the entire length of said strip material in addition to the forming and bending pressures mechanically induced by the working faces of said dies; and c. pulling said strip material continuously through the orifices of said dies to form the strip into the point generatrix crosssectional shape, said strip passing sequentially from said die orifices most closely approximating the initial shape of said strip to said die substantially of said point generatrix shaped cross section while liquid hydrostatic pressure is applied to the entire length of said strip while being continuously formed into said shape of point generatrix cross section
 2. The method of claim 1 wherein at least one die of substantially said point generatrix shape is disposed to effect a draw reduction not more than 2 percent.
 3. The method of claim 1 wherein said workpiece consists of a strip of metal and said dies are disposed to sequentially form said strip into a sharp angled structural member of L, V, U, Z, or hat-shaped configuration.
 4. An apparatus for forming and bending strip material into an article with a point generatrix cross section comprising: a. a chamber; b. a plurality of dies of varying orifice shape ranging between the initial cross-sectional shape of said strip material to and including said point generatrix cross section aligned within said chamber, said alignment being sequential from the die having an orifice most closely approximating the initial cross-sectional shape of said strip material to a die substantially of said point generatrix cross-sectional shape; c. means for the orifices of the end dies of said alignment to communicate through the walls of said chamber so that said strip material may be pulled through said chamber and said die orifices; d. means for providing hydrostatic pressure within said chamber to subject said strip material to said hydrostatic pressure along its entire length in addition to the mechanically induced forming and bending pressures exerted by the working faces of said die orifices while being pulled through said chamber and orifices and sequentially formed into said point generatrix cross-sectional shape; and e. means for pulling said strip material through said chamber and said plurality of dies to form said strip into the point generatrix cross-sectional shape while liquid hydrostatic pressure is applied to the entire surface of said strip material in addition to the mechanical forming and bending forces exerted by said dies.
 5. The apparatus of claim 4 wherein said means for providing liquid hydrostatic pressure within said chamber is capable of providing said pressure within the range of from about 5,000 p.s.i. to 400,000 p.s.i.
 6. An apparatus as set forth in claim 4 wherein at least one of said dies is disposed to effect a draw reduction on said point generatrix shape of not more than 2 percent. 